Generally, the invention relates to the development of a coating technology to apply different compositions of refractory materials such as those containing hard metals, particularly titanium borides, metallic alloys, intermetallic compounds, cermets, oxides, metals and ceramics to the surface of substrates made of different materials such as carbonaceous materials, refractory materials, ceramics, cermets, oxides, metallic alloys (particularly those of iron, nickel, aluminum, and copper) and intermetallic compounds.
Such substrates may in particular be components of electrolytic cells operating at high temperatures, particularly aluminium production cells. The present invention thus more specifically relates to a novel method of application of adherent protective coatings of refractory material to the surface of substrates of components of electrolytic cells for molten salt electrolysis for the electrowinning of metals and operating at high temperatures, particularly for the production of aluminium and as well to novel designs of such cells and their operation.
The protective coating is a refractory material or a combination of refractory materials containing aluminum-wettable hard metals, particularly titanium borides or other materials consisting of metallic alloys, intermetallic compounds, cermets, oxides and ceramics on the surface of the substrates e.g. of electrolytic cell components, in particular an adherent protective coating of aluminium-wettable refractory material on the surface of a carbonaceous or refractory substrate lining the cell bottom floor of an aluminium production cell.
The invention also relates to composite materials comprising a carbonaceous or refractory substrate coated with an aluminium-wettable refractory material and to the use of the coated composite materials in such cells.
Among the metals obtained in electrolytic cells operating at high temperature in a molten salt electrolyte containing an oxide or compound of the metal to be electrowon, aluminium is the most important and the invention will describe in particular the protection of components of aluminium cells, more particularly the protection of the cell cathode bottom by applying an aluminium wettable, adherent coating.
Aluminium is produced conventionally by the Hall-Hxc3xa9roult process, by the electrolysis of alumina dissolved in molten salt containing cryolite at temperatures around 950xc2x0 C. A Hall-Hxc3xa9roult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Hxc3xa9roult cells, a molten aluminium pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In additon, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact and an excessive iron content in the aluminium metal produced.
A major drawback of carbon as cathode material is that it is not wetted by aluminium. This necessitates maintaining a deep pool of aluminium (at least 100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminium and, to avoid short-circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminium pool and the anode. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals (RHM) such as TiB2 as cathode materials. TiB2 and other RHM""s are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the saving of energy by reducing the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
The non-acceptance of tiles and other methods of applying layers of TiB2 and other RHM materials on the surface of aluminium production cells is due to their lack of stability in the operating conditions, in addition to their cost. The failure of these materials is associated with penetration of the electrolyte when not perfectly wetted by aluminium, and attack by aluminium because of impurities in the RHM structure. In RHM pieces such as tiles, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminium metal and/or by cryolite. To combat disintegration, it has been proposed to use highly pure TiB2 powder to make materials containing less than 50 ppm oxygen. Such fabrication further increases the cost of the already-expensive materials. No cell utilizing TiB2 tiles as cathode is known to have operated for long periods without loss of adhesion of the tiles, or their disintegration. Other reasons for failure of RHM tiles have been the lack of mechanical strength and resistance to thermal shock.
Various types of TiB2 or RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the titanium diboride material and the carbon cathode block.
U.S. Pat. No. 3,400,061 describes a cell without an aluminium pool but with a drained cathode of Refractory Hard Metal which consists of a mixture of Refractory Hard Metal, at least 5 percent carbon, and 10 to 20% by weight of pitch binder, baked at 900xc2x0 C. or more and rammed into place in the cell bottom. Such composite cathodes have found no commercial use probably due to susceptibility to attack by the electrolytic bath.
U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride and other Refractory Hard Metals to a conductive substrate such as graphite. But large differences in thermal expansion coefficients between the RHM tiles and the substrate cause problems.
U.S. Pat. No. 3,661,736 claims a composite drained cathode for an aluminium production cell, comprising particles or pieces of arc-melted xe2x80x9cRHM alloyxe2x80x9d embedded in an electrically conductive matrix of carbon or graphite and a particulate filler such as aluminium carbide, titanium carbide or titanium nitride. However, in operation, grain boundaries and the carbon or graphite matrix are attacked by electrolyte and/or aluminium, leading to rapid destruction of the cathode.
U.S. Pat. No. 4,308,114 discloses a cathode surface of RHM in a graphitic matrix made by mixing the RHM with a pitch binder and graphitizating at 2350xc2x0 C. or above. Such cathodes are subject to early failure due to rapid ablation, and possible intercalation by sodium and erosion of the graphite matrix.
To avoid the problems encountered with tiles and with the previous coating methods, U.S. Pat. No. 4,466,996 proposed applying a coating composition comprising a preformed particulate RHM, such as TiB2, a thermosetting binder, a carbonaceous filler and carbonaceous additives to a carbonaceous cathode substrate, followed by curing and carbonisation. But it is still not possible by this method to obtain coatings of satisfactory adherence that could withstand the operating conditions in an aluminium production cell. It has also proven impossible to produce adherent coatings of RHM on refractory substrates such as alumina.
U.S. Pat. No. 4,560,448 describes a structural component of an aluminium production cell which is in contact with molten aluminium, made of a non-wettable material such as alumina which is rendered wettable by a thin layer (up to 100 micrometer) of TiB2. However, to prevent dissolution of this TiB2 layer, the molten aluminium had to be maintained saturated with titanium and boron and this expedient was not acceptable.
U.S. Pat. No. 5,004,524 discloses a body of fused alumina or another refractory oxycompound having a multiplicity of discrete inclusions of TiB2 or other aluminium-wettable RHM cast into its surface. This material is particularly suitable for non-current carrying cathode bottom floors of aluminium production cells, but in the long term even if the material may remain bound to the fused alumina and resist to corrosion, the manufacture at an acceptable cost remains a problem.
U.S. Pat. No. 4,595,545 discloses the production of titanium diboride or a mixture thereof with a carbide and/or a nitride of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum or tungsten by carbothermic, carbo-aluminothermic or alumino-thermic reaction, under vacuum or an inert atmosphere, of a glass or microcristalline gel of oxide reactants prepared from organic alkoxide precursors. This glass or gel was then ground and formed into bodies and sintered into bodies of titanium diboride/alumina-based materials as components of aluminium production cells. But such sintered materials are subject to attack and grain-boundary corrosion when in contact with molten aluminium. Similar reactions, known as combustion synthesis, self-propagating high temperature synthesis or micropyretic reactions are known (see below, under the heading xe2x80x9cMicropyretic Reactionsxe2x80x9d), but to date these reactions have not been applied to the production of refractory coatings on carbonaceous, refractory or other substrates in such a way, and with the right composition, as to lead to coatings with adequate adherence to survive the operating conditions in an aluminium production cell.
U.S. Pat. No. 4,600,481 proposed making components of aluminium production cells by infiltrating aluminium into a skeletal self-sustaining matrix of alumina or another refractory material which is normally non-wettable by molten aluminium, after having rendered the surface of the matrix wettable by molten aluminium for instance by treating the surface with a wetting agent such as titanium diboride, in particular with a titanium diboride composite material produced according to the previously-mentioned patent. In this case, only a temporary surface wetting was thought to be required to facilitate the infiltration, but in practice it was not easy to produce materials that sufficiently maintained the internal wetting to sustain long operating periods when the component was exposed externally to molten aluminium. Also, the described techniques have not been applied to external surfaces of refractory bodies to make them permanently wettable by molten aluminium.
The methods employed to date have thus not successfully produced adherent protective coatings of refractory materials, in particular aluminium wettable refractory materials such as TiB2 and other Refractory Hard Metals, on various substrates and in particular on carbonaceous or refractory substrates, that adhere to and remain firmly attached to the substrate in conditions such as encountered in aluminium production cells, the coating providing a permanent and perfectly protective surface that is wetted by molten aluminium.
The invention aims to overcome the deficiencies of past attempts to utilize refractory materials in particular Refractory Hard Metals as surface coatings on substrates, in particular but not exclusively carbonaceous, refractory and metallic substrates, for use generally for protecting the substrates from the corrosive attacks of liquids and gases, inter alia for use as cell components for molten salt electrolysis cells, especially for use as cathodes or other cell components of aluminium production cells.
The invention relates in particular to the protection of the surfaces of components of electrolytic cells, particularly those operating at high temperatures, from the attack of liquids and gases existing in the cells or formed during electrolysis by applying a refractory coating by utilizing novel micropyretic methods. A refractory coating or refractory material when mentioned in this description of the invention shall mean a material, whether carbonaceous, ceramic, or metallic, which can withstand high temperatures.
An object of the invention is to provide a method of producing refractory materials, in particular aluminium wettable refractory materials, making use of a micropyretic reaction in a slurry-applied reaction layer of such composition and so controlled that the method can produce extremely adherent refractory coatings on carbonaceous, refractory, metallic or other substrates that can inter alia be used as cathodes in aluminium production or more generally as any cell component where wettability with aluminium is desirable, as well as resistance to cryolite and oxidation. Other applications may make use of the material""s excellent resistance to corrosion, in particular to oxidation, especially in high temperature environments.
The coating is obtained by applying to the surface of the substrate, e.g. of the component of the electrolytic cell which needs to be coated and protected, a well chosen micropyretic slurry which when dried is ignited to initiate a self-sustaining micropyretic reaction in the dried slurry, along a combustion front, to produce condensed matter forming a coating adherent to the surface of the substrate and protecting it.
The composition of the micropyretic slurry is chosen according to the physical and chemical characteristics of the substrate and the purpose of the coating. The slurry is preferably applied in several layers, the first layer(s) to facilitate adherence and the last layer(s) to provide protection.
The coatings obtained by the method according to the invention are well adherent to the different substrates, provide the required protection to the cell components and have the desired mechanical, physical, chemical, and electrochemical characteristics.
The coatings are impervious and adherent to the substrates and resistant to thermal shocks therefore protecting the substrates efficiently from the corrosive attacks of liquids, fumes and gases existing or produced in electrolytic cells, thus making them ideal for use in molten salt electrolysis cells, in particular those for aluminum production. In an electrolytic cell operating at high temperature all cell components have to be mechanically strong at the operating temperature and each one may have any additional required characteristic.
In the particular case of aluminium production cells, an aluminium-wettable, refractory, electrically conductive, adherent coating has been developed to be applied to the surface of the cell cathode bottom made of carbonaceous material to protect such carbonaceous material from the attack of sodium and air which produces deformation of the cathode blocks and formation of dangerous nitrogen compounds such as cyanides.
By protecting the carbonaceous cell components from attack by NAF or other aggressive ingredients of the electrolyte, the cell efficiency is improved. Because NaF in the electrolyte no longer reacts with the carbon cell bottom and walls, the cell functions with a defined bath ratio without a need to replenish the electrolyte with NaF.
The aluminum-wettable refractory coating will also permit the elimination of the thick aluminium pool required to partially protect the carbon cathode, enabling the cell to operate with a drained cathode. Other coatings have been developed to protect the upper part of the carbonaceous cell wall and cell cover and anode current feeders and holders from the attack of fluoride fumes and oxidation by oxygen or air and the lower part from the attack by the cryolite-containing electrolyte.
Special coatings have also been developed to protect anode substrates from the attack of oxygen and cryolite.
The protective effect of the coatings according to the invention is such as to enable the use of relatively inexpensive materials for the substrates. For instance, cheaper grades of graphite can be used instead of the more expensive anthracite forms of carbon, while providing improved resistance against the corrosive conditions in the cell environment.
The composite materials resulting from coating substrates according to the present invention can be utilized also as components of electrolytic cells for the production by molten salt electrolysis of other metals such as magnesium, sodium, potassium, titanium, and others, and also for cells operating at low temperatures and for the surfaces of any other parts of electrochemical equipment requiring electrochemical, chemical, or physical stability.
The present invention concerns a method which is not only superior and less costly than other suggested, well-known methods such as plasma or flame spray, electrodeposition and dip coating, but in many cases is the only applicable and efficient method.
According to the invention, a method has been developed for producing a component of an aluminium production cell which in operation of the cell is exposed to a molten electrolyte and/or to molten aluminium, which component comprises a substrate of carbonaceous or refractory material or a cermet, a metal, a refractory oxide, a metallic alloy or an intermetallic compound coated with a coating of refractory material. This method comprises applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants preferably in a colloidal carrier, and initiating a micropyretic reaction. More specifically, the invention relates to a method of producing a refractory adherent material by applying one or more layers of one or more micropyretic slurries one or more of which contains particulate reactants, to a substrate and drying each of them before applying the following layer, to provide on the substrate at least one dried layer containing the particulate reactants. The slurry-applied layer is then ignited to initiate a self-sustaining micropyretic reaction in the dried layer, along a combustion front, to produce condensed matter forming a coating of refractory material adherent to the surface of the substrate and protecting it.
To assist rapid wetting of the components by molten aluminium, the refractory material coated on the substrate may be exposed to molten aluminium in the presence of a flux assisting penetration of aluminium into the refractory material, the flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof. Such treatment favors aluminization of the refractory coating by the penetration therein of aluminium. Aluminization may also be assisted by including powdered aluminium in the slurry of micropyretic reactants with optional non-reactive fillers.
The substrate of the component may be coated outside the aluminium production cell and the coated component then inserted into the cell. Alternatively, the component is part of a cell which is coated in the cell prior to operation. For instance, the component is part of a cell bottom formed by an exposed area of carbonaceous material, an exposed area of refractory material, an exposed area of a metal alloy, or an expanse comprising exposed areas of carbonaceous material, refractory material and/or metal alloys. In this case, the slurry is preferably applied to the cell bottom in several layers with drying of each successive layer, and the micropyretic reaction is initiated by a mobile heat source. The micropyretic slurry preferably contains the particulate reactants in a colloidal carrier, e.g. comprising colloidal silica, colloidal yttria, and/or colloidal monoaluminium phosphate in various solvents. This colloidal carrier may be in an aqueous solvent but advantageously comprises an organic solvent, particularly an urethane-based solvent.
Particulate or fibrous non-reactant filler materials can be included by applying one or more layers from a slurry of particulate non-reactant filler materials or by including particulate or fibrous non-reactants in the micropyretic slurry.
The substrate may be carbonaceous in which case it may be made of anthracite based carbon or of graphite and other grades of carbon used in aluminium production cells. Advantageously, use may be made fo the cheaper grades of carbon. Ceramic substrates include but are not limited to alumina and other materials that are not normally wettable by molten aluminium, such as aluminium nitride, aluminium oxynitride, boron nitride, silicon carbide, silicon nitride and aluminium boride. Other ceramics, cermets, metals such as copper and metallic alloys such as steel and cast iron or those of nickel, aluminium and copper can also serve successfully as substrates utilizing the present invention. The substrates may be bodies or tightly packed agglomerates. The substrates may have a microporous surface providing anchorage for the applied aluminium-wettable refractory material. Thus, sintered or tightly packed substrates may sometimes be preferred over highly dense materials such as solid blocks of fused alumina.
It is also possible, according to this invention, to apply the coating from a micropyretic slurry onto a skeletal substrate as taught in U.S. Pat. No. 4,600,481, to produce an adherent and permanent refractory aluminium-wettable coating throughout the skeletal substrate.
The substrate may consist of blocks that can be fitted together to form a cell bottom of an aluminium production cell, or packed particulate material forming a cell bottom. When a carbonaceous substrate is used, it will act to carry current to the cathodic pool if there is one, or to a thin layer of aluminium through the refractory coating in drained cells. When a refractory substrate is used, the aluminium-wettable refractory coating assists in maintaining a shallow pool of molten aluminium which needs to be only deep enough to permit good current distribution.
In this case separate current conductors are provided through the refractory cell bottom for the supply of current, e.g. as disclosed in U.S. Pat. No. 5,071,533 with the possible improvement that the tops and sides of the current feeders may also be coated with refractory material as disclosed herein.
Steel, cast iron or other metallic alloy substrates, coated according to the invention with a refractory coating, can be used as cathodic current feeders extending through a refactory bottom of an aluminium production cell or can be coated with a refractory coating suitable for anodic applications.
The micropyretic slurry which is the precursor of the aluminium-wettable refractory coating may be applied in one or more layers directly to the substrate or onto a non-micropyretic sub-layer applied in one or more layers on the surface of the substrate.
The non-micropyretic sub-layer may be one or more coatings of a slurry of particulates of pre-formed materials compatible with the substrate and with the aluminium-wettable refractory coating. In particular, the sub-layer may contain pre-formed aluminium-wettable refractory material which is the same as that in the aluminium-wettable refractory coating, and it may also contain other refractory additives which may also be present in the aluminium-wettable refractory coating. Thus, the non-micropyretic under or bottom layer(s) may be produced by applying a slurry similar to the micropyretic slurry, except that it does not contain the micropyretic reactants
The invention also concerns a component of an aluminium production cell which in use is subjected to exposure to molten electrolyte and/or to molten aluminium or corrosive fumes or gases, the component comprising a substrate of a carbonaceous, ceramic or metallic material, a cermet, or a compound coated with a refractory material comprising at least one boride, silicide, nitride, carbide phosphide, aluminide or oxide of at least one of titanium, zirconium, hafnium, vanadium, silicon, niobium, tantalum, nickel, molybdenum and iron or mixtures thereof, finely mixed with a refractory compound of at least one rare earth, in particular ceria or yttria, possibly together with other refractory oxycompounds such as alumina or oxides, nitrides, carbides, suicides, aluminides of at least one of the above-listed elements or silicon, as such or in colloidal form.
The preferred refractory coatings have the following attributes: excellent wettability by molten aluminium, excellent adherence to many different substrates, inertness to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks without delamination from the anthracite-based carbon or other substrates, durability in the environment of an aluminium production cell, and ease of application and processing. The coatings furthermore have a controlled microporosity depending on the size of the particulate non-reactants as well as the thermal conditions during the micropyretic reaction along the combustion front.
When these refractor coatings are applied to a substrate, for instance of graphite or anthracite-based carbon, refractory material or steel used in an aluminium production cell in contact with the molten electrolyte and/or with molten aluminium, the coating protects the substrate against the ingress of cryolite and sodium and is in turn protected by the protective film of aluminium on the coating itself.
The invention also relates to an aluminium production cell comprising a coated component as discussed above as well as a method of producing aluminium using such cells and methods of servicing and/or operating the cells.
A method of operating the cells comprises
producing a cell component which comprises a substrate of carbonaceous or refractory material or a metallic alloy and a protective coating of refractory material, by applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants preferably in a colloidal carrier, and initiating a micropyretic reaction;
if the micropyretic reaction is initiated and its preparation completed outside the cell, placing the coated component in the cell so the coating of refractory material will be contacted by the cathodically produced aluminium, and/or the molten electrolyte, and/or the anodically-released gases; and
operating the cell with the coating protecting the substrate from attack by the cathodically-produced aluminium, by the molten electrolyte and by the anodically-released gases with which it is in contact.
The component may be a current-carrying component made of metal, metal alloy, or an intermetallic compound, for example a cathode, a cathode current feeder, an anode or an anode current feeder. Or the component may be a bipolar electrode coated on its cathode face, or on its anode face, or both.
In operation of the cell the component may be exposed to corrosive or oxidising gas released in operation or present in the cell operating conditions, such component comprising a substrate of carbonaceous material, refractory material or metal alloy that is subject to attack by the corrosive or oxidising gas and a coating of refractory material protecting it from corrosion or oxidation.
It is advantageous for the component to have a substrate of low-density carbon protected by the refractory material, for example if the component is exposed to oxidising gas released in operation of the cell, or also when the substrate is part of a cell bottom. Low density carbon embraces various types of relatively inexpensive forms of carbon which are relatively porous and very conductive, but hitherto could not be used successfully in the environment of aluminium production cells on account of the fact that they were subject to excessive corrosion or oxidation. Now it is possible by coating these low density carbons according to the invention, to make use of them in these cells+ instead of the more expensive high density anthracite and graphite, taking advantage of their excellent conductivity and low cost.
The component advantageously forms part of a cathode through which the electrolysis current flows, the refractory coating forming a cathodic surface in contact with the cathodically-produced aluminium. For example, it is part of a drained cathode, the refractory coating forming the cathodic surface on which the aluminium is deposited cathodically, and the component being arranged usually upright or at a slope for the aluminium to drain from the cathodic surface.
Operation of the cell is advantageously in a low temperature process, with the molten halide electrolyte containing dissolved alumina at a temperature below 900xc2x0 C., usually at a temperature from 680xc2x0 C. to 880xc2x0 C. The low temperature electrolyte may be a fluoride melt, a mixed fluoride-chloride melt or a chloride melt.
This low temperature process is operated at low current densities on account of the low alumina solubility. This necessitates the use of large anodes and corresponding large cathodes, exposing large areas of these materials to the corrosive conditions in the cell, such large exposed areas being well protected by the refractory coatings according to the invention which are just as advantageous at these lower temperatures.
The refractory coatings find many applications on account of their excellent resistance, protection, and stability when exposed to the corrosive action of liquids and fumes existing in the cell or formed during electrolysis even when the temperature of operation is low as in the Low Temperature electrolysis process for the production of aluminium (see for example U.S. Pat. No. 4,681,671).
The invention is based on the use of a micropyretic slurry, which when ignited starts a micropyretic reaction.
Micropyretic reactions are already known. A micropyretic reaction is a sustained reaction with formation of condensed matter, starting with finely divided particulate reactants which during the reaction are in solid state or in suspension in a liquid. The combustion takes place without a gaseous reactant and usually without gaseous reaction products. The reactants are most often in elemental form, but may be compounds, eg. nitrides, when nitrides are desired in the reaction products. Micropyretic reactions are exothermic and can be initiated in a point or zone ignited by bringing the reactants to the reaction temperature. In micropyretic reactions, ignition starts a sustained reaction with formation of the condensed matter, this sustained reaction proceeding along a combustion front whose propagation can be controlled by choice of the reactants, the non-reactants or fillers and the carriers, which are the liquid portion of the slurry. Such reactions are self-propagating and are sometimes known in the literature as combustion synthesis (CS) or self-propagating high-temperature synthesis (SHS). Two modes of micropyretic heating reaction are recognized. One where heating is at one point and propagation is very apparent (called the self-propagating mode), the other where propagation needs assistance (called the thermal explosion mode).
Almost all known ceramic materials can be produced by combustion synthesis, but not necessarily without unwanted impurities. It has been pointed out that considerable research is needed and that major difficulties are encountered in achieving high product density and adequate control over the reaction products (see for example H. C. Yi et al in Journal Materials Science, 25, 1159-1168 (1990)).
SHS techniques using pressed powder mixtures of titanium and boron; titanium, boron and titanium boride; and titanium and boron carbide have also been described (see J. W. McCauley et al, in Ceramic Engineering and Science Proceedings, 3, 538-554 (1982)).
Reactions using titanium powders to produce TiC, TiB2 or TiC+TiB2 have also been studied. The compact density of the reactant powder was found to be a major factor in the rate of reaction propagation (see R. W. Rice et al, Ceramic Engineering and Science Proceedings, 7, 737-749, (1986)).
U.S. Pat. No. 4,909,842 discloses the production by SHS of dense, fine-grained composite materials comprising ceramic and metallic phases, by the application of mechanical pressure during or immediately after the SHS reaction. The ceramic phase of phases may be carbides or borides of titanium, zirconium, hafnium, tantalum or niobium, silicon carbide, or boron carbide. Intermetallic phases may be aluminides of nickel, titanium or copper, titanium nickelides, titanium ferrides, or cobalt titanides. Metallic phases may include aluminium, copper, nickel, iron or cobalt. By applying pressure during firing, the final product of ceramic grains in an intermetallic and/or metallic matrix had a density of about 95% of the theoretical density.
Known micropyretic reactions by CS or SHS are not without drawbacks and are inadequate to produce adherent refractory coatings on carbonaceous, refractory or other substrates, in particular for use as cell components in aluminium production, which the invention has succeeded in producing, starting from micropyretic slurries of special composition as described herein.
The application of micropyretic reactions to produce net-shaped electrodes for electrochemical processes, in particular for aluminium production, is the subject of U.S. Pat. Nos. 5,217,583 and 5,316,718, the contents of which are incorporated herein by way of reference. In said applications, a mixture of particulate or fibrous combustion synthesis reactants with particulate or fibrous filler materials and a particulate or fibrous, non-reactant, inorganic binder is used to produce a bulk electrode by combustion synthesis.
The present invention provides unexpectedly good results by using a novel micropyretic slurry of particulate reactants possibly with particulate or fibrous diluents and non-reactant filler materials which is advantageously applied to a carbonaceous, refractory or metallic substrate before initiating the reaction. This slurry when ignited starts a self-sustaining reaction, along a combustion front, to produce the refractory material, the components of the slurry and the refractory material produced forming condensed matter along the combustion front as the reaction proceeds. The produced refractory material is usually selected from the group of borides, silicides, nitrides, carbides, phosphides, aluminides or oxides, and mixtures thereof, of at least one metal selected from titanium, zirconium, hafnium, vanadium, silicon, niobium and tantalum, nickel, molybdenum, chromium and iron, as well as metal alloys, intermetallic compounds, cermets or other composite materials based on said metal or mixtures thereof or mixtures with at least one of the aforesaid compounds. The refractory borides of titanium, zirconium, hafnium, vanadium, niobium and tantalum, or combinations thereof with the other listed materials are preferred.
The micropyretic slurry comprises particulate micropyretic reactants in combination with optional particulate of fibrous non-reactant fillers or moderators in a carrier of colloidal materials or other fluids such as water or other aqueous solutions, organic carriers such as acetone, urethanes, etc., or inorganic carriers such as colloidal metal oxides.
The colloidal carrierxe2x80x94usually colloidal alumina, colloidal silica, colloidal yttria or colloidal monoaluminium phosphate and usually in an aqueous mediumxe2x80x94has been found to assist in moderating the reaction and considerably improve the properties of the coating. It is however not necessary for all of the applied layers of the slurry to have a colloidal carrier. Excellent results have been obtained using some slurries with a colloidal carrier and others with an organic solvent. Combinations of a colloidal carrier in aqueous medium and an organic solvent have also worked well.
The micropyretic combustibles may comprise components to produce, upon reaction, borides, silicides, nitrides and aluminides, and mixtures thereof, of titanium, zirconium, hafnium, vanadium, silicon, niobium and tantalum, nickel, molybdenum, chromium and iron. Mostly, these reactants will be in the elemental form, but may also be compounds, for example for the production of nitrides. The reactants are preferably finely divided particulates comprising elements making up the aluminium-wettable refractory material produced. The reactants are preferably in the stoichiometric proportions necessary to produce the desired end products without leaving any residual reactants.
Titanium diboride will henceforth be described by way of example as the final material, starting from elemental particulate titanium and boron in equimolar proportions in the micropyretic reaction slurry. It will readily be understood that other refractory compounds and mixtures can be produced in similar manners by using the appropriate starting reactants and adjusting the parameters of the production process.
The micropyretic reaction slurry may also comprise non-reactant fillers such as pre-formed particulates or fibers of the desired refractory material being produced, for instance, pre-formed particulate titanium diboride together with elemental titanium and boron. Other inert fillers which may be desirable to moderate the micropyretic reaction and/or to enhance the properties of the end product may also be included.
Such fillers thus are advantageously included in combination with colloids in a liquid carrier for the reactants, such as colloidal alumina, colloidal yttria, colloidal ceria, colloidal phosphates in particular colloidal monoaluminium phosphate, or colloidal silica. More generally, colloids of other elements may be included, alone or in combination. These products do not take part in the reaction, but serve as moderators, and contribute to the desired properties of the end product. All of these colloids act as carriers for the particulate micropyretic combustible slurry or for the non-reactant slurry.
The solvent of the carrier for the reactant or non-reactant slurry may be an organic solvent in particular a urethane-based solvent such as polyurethane, acetone but also water or aqueous solutions, possibly together with monoaluminium phosphate.
Other organic solvents, especially for use in combination with colloids include isopropanol, ethyleneglycol, dimethylacetonide and mono-n-propylether.
The use of organic solvents which are carbonised during the micropyretic reaction can be particularly advantageous on carbonaceous substrates, eg. due to the formation of glassy or vitreous carbon which assists bonding of the coating to the carbonaceous substrate. Organic materials suitable for producing glassy carbon include polyurethane/furan resins, polyacrylonitrile, cellulose pitch, vinyl alcohol, thermosetting resins, etc. Other usable polymers include polyacrylamide and other derivatives of polyacrylic acid, soluble aromatic polymers such as aromatic polyamides, aromatic polyesters, polysulfanes, aromatic polysulfides, epoxy, phenoxy or alkyde resins containing aromatic building blocks, polyphenylene or polypheyhlene oxides. Heteroaromatic polymers such as polyvinylpyridine, polyvinylpyrrolidone or polytetrahydrofurane can also be used as well as prepolymers convertible to heteroaromatic polymers, for instance polybenzoyazotes or polybenzimidazopyrrolones. Polymers containing adamantane, especially the above-mentioned prepolymers containing adamantane units, may also be used. For instance, polybenzimidazopyrrolidone (pyrrone) and adamantane based polybenzoxyzote (PBO) can be used in a solution of N-methyl pyrrolidone. Such polymers are pyrolised during the micropyretic reaction to form semiconductive polymers and/or glassy forms of carbon, which adhere especially well to carbonaceous substrates although excellent results may be obtained too on other substrates such as ceramic or metallic.
Surprisingly, when using organic solvents, superior results have been obtained when the slurry with the organic solvent is applied on top of one or more underlayers of a slurry with a non-organic solvent, usually one containing a colloidal carrier.
The components of the slurry thus consist of the particulate reactants, optional particulate or fibrous fillers and the carrier which is usually a colloidal carrier and which may, eg. as in the case of monoaluminium phosphate and organic carriers, be transformed or react during the micropyretic reaction.
The particulates usually have a maximum dimension not exceeding about 100 micrometers, more often 50 microns or less. The fillers can be particulates of similar dimensions, or may be fibrous in which case they may be larger than 100 microns.
The colloids are submicronic; their particles are of the order of a nanometer.
It has been found that when well-chosen slurries are ignited after drying, a controlled micropyretic sustained reaction takes place to produce an intimate mixture of the resulting reaction product with the fillers and the carriers, e.g. titanium diboride or other refractory compounds with desirable quantities of aluminium, ceria, yttria, alumina, silica or other materials including glassy carbon or other forms of carbon which do not detract from the wettability of the material by molten aluminium, but usually improve the adherence and the protection. Such materials are particularly advantageous when formed as coatings on a carbonaceous or ceramic substrate, though, as mentioned, adherent, protective coatings can be applied also to metallic substrates.
The production of a refractory material as a coating on carbonaceous, ceramic, metallic or other substrates involves the application of the micropyretic slurry of particulate reactants, particularly in a colloidal carrier, alone or along with particulate or fibrous fillers, directly on the substrate or onto a non-reactive sub-layer or sub-layers devoid of particulate micropyretic reactants but which may include a pre-formed particulate of the refractory material being produced and/or other particulate or fibrous non-reactants. The sub-layer(s) is/are preferably also applied with its or their particulates suspended in a colloidal carrier in an aqueous or organic solvent.
The reactant coating may be formed by applying one or more layers of the micropyretic slurry each from about 50 to 1000 micrometers thick, each coating being followed by drying before applying the next layer. The same applies to the sub-layer which can be built up by applying successive coatings, each followed by an at least partial drying. Application in multiple layers improves the strength of the coating after drying and before combustion (the so-called green strength), and this leads to better properties in the end product including uniform, controlled pore size and distribution and greater imperviousness.
These layers can be formed by any convenient technique including painting, dipping, spraying and slip-casting. The drying can be carried out by air drying at ambient temperature or above, or by pre-heating the substrate, and possibly in an atmosphere with controlled humidity.
It is also possible to apply successive layers of the slurry containing the particulate reactants, possibly mixed with particulate or fibrous non-reactants, and layers of the slurry containing the particulates or fibrous non-reactants in a multilayer sandwich. Preferably, a reactant layer will be on top, but it is also possible to top-coat with slurries containing pre-formed refractory material.
When all of the layers have been applied, it is important to allow the coatings to dry for a prolonged period to provide coatings without cracks and with adequate green strength and to eliminate water and/or other low boiling point solvents. This full drying may take place in air for several hours to several days, depending mainly on the temperature and the humidity of the air and on the total thickness of the coatings which may range from about 100 micrometers to about 3000 micrometers and more.
The combustion reaction can then be initiated by wave propagation or by a thermal explosion mode. In the wave propagation mode, the reaction is started from one part of the completed green coating and propagates through the entire green surface. It may be advantageous to heat the surface of the coating to a preheat temperature for instance from between 200xc2x0 C. and 500xc2x0 C. In the thermal explosion mode, the reaction is started by heating the entire surface of the coating and possibly the substrate to the required temperature to initiate the combustion reaction at all locations in the coating.
Usually the wave propagation mode is more practical. This uses a torch, laser, plasma, the passage of electric current, or any other suitable mobile heat source to initiate the micropyretic reaction and to help sustain the reaction if necessary, or that can be moved over the coating at a desired scanning rate to progressively initiate the reaction over the coating as the heat source passes by. The thermal explosion mode can employ an induction furnace or other conventional means such as another type of furnace or radiant heater, and this may give better control of the reaction leading to more homogeneous properties.
The ignition temperature is usually in the range 500-2000xc2x0 C., depending on the reactants. Combustion may be preceded by preheating for an adequate time, about 10-60 seconds in some cases and an hour or more in others.
The micropyretic reaction may take place in air but advantageously takes place in a reducing atmophere containing, for example, CO2.
In the case of the wave propagation mode, combustion progresses along a front parallel to the surface of the substrate being coated. The temperature reaches a peak at the combustion front. Ahead of the combustion front, the uncombusted part of the reactants is at a relatively low temperature. Behind the combustion front, the temperature drops gradually.
In the thermal explosion mode, the combustion reaction is started at all locations of the coating, and progresses rapidly in depth through the coating.
The finished material obtained utilizing slurries of well chosen composition and methods according to the invention adheres perfectly to the substrate, due to the controlled progression of the reaction front during the micropyretic reaction and the choice of the first layer(s) of the slurry.
Particularly for carbonaceous substrates, it is advantageous for at least the bottom layer or the adjacent under layer(s) of the slurry coating to have an organic carrier which, when subjected to the heat treatment during the micropyretic reaction, is pyrolized to carbon bonding the resultant coating to the substrate.
For alumina and other ceramic or metallic substrates, excellent adhesion of the coating is obtained in a similar manner, since the coatings penetrate into pores on the surface or between particles of the substrate and become anchored therein.